In an impulse radio communication system, for example, communications are generally carried out by use of a pulse signal. In “Impulse radio: how it works”, Win, M. Z.; Scholtz, R. A, IEEE Communications Letters, February 1998, Volume: 2 Issue: 2 (hereinafter, referred to as “Non-patent document 1”), there is disclosed a communications system of PPM (Pulse Position Modulation) method, which modulates a Gaussian Monocycle Pulse at a pulse position. Such communications method as described above, which uses a pulse train, carries out transmission of information by sending and receiving intermittent energy signals, unlike an ordinary signal transmission using a continuous wave.
When a pulse constituting the pulse train has an extremely narrow pulse width, a signal spectrum thereof may cause a larger frequency band, thereby dispersing energy of signal, compared to the ordinary communications using the continuous wave. Consequently, the signal energy in each frequency becomes very minute, and communications can be carried out without interference with other communications system. Therefore, it is possible to share the frequency band.
However, assuming a case of wireless communications, for example, external environment around wireless equipment is constantly changing due to movement of the communications apparatus, shifting of an object and/or a person, and the like. Therefore, there has been a problem that a communication quality is forced to be varied in accordance with the external environmental change.
In order to ensure a good communication quality even in the case where the communication environment is deteriorated, there is disclosed a method which measures a communication environment and based on the result thereof it controls a transmission power, as disclosed in the Japanese unexamined patent publication No. 2003-51761 (hereinafter, referred to as “Patent document 1”) and the specification in U.S. Pat. No. 6,539,213 (hereinafter, referred to as “Patent document 2”).